Nonlinear pulsations of red supergiants

TL;DR

This paper investigates the radial pulsations of red supergiants, showing their instability due to the kappa-mechanism, and proposes a method to estimate their masses from pulsation periods.

Contribution

It provides new insights into the pulsation mechanisms of red supergiants and introduces an approximate method to determine their masses from observed periods.

Findings

Red supergiants with 8-20 solar masses are unstable to radial pulsations.

Pulsation periods range from 45 to 1180 days.

An approximate method to estimate stellar mass from pulsation period is proposed.

Abstract

Excitation of radial oscillations in population I (X=0.7, Z=0.02) red supergiants is investigated using the solution of the equations of radiation hydrodynamics and turbulent convection. The core helium burning stars with masses 8M_odot <= M <= 20M_odot end effective temperatures T_eff < 4000K are shown to be unstable against radial pulsations in the fundamental mode. The oscillation periods range between 45 and 1180 days. The pulsational instability is due to the kappa-mechanism in the hydrogen and helium ionization zones. Radial pulsations of stars with mass M < 15M_odot are strictly periodic with the light amplitude Delta M_bol <= 0.5 mag. The pulsation amplitude increases with increasing stellar mass and for M > 15M_odot the maximum expansion velocity of outer layers is as high as one third of the escape velocity. The mean radii of outer mass zones increase due to nonlinear oscillations by <= 30% in comparison with the initial equilibrium. The approximate method (with uncertainty of a factor of 1.5) to evaluate the mass of the pulsating red supergiant with the known period of radial oscillations is proposed. The approximation of the pulsation constant Q as a function of the mass-to-radius ratio is given. Masses of seven galactic red supergiants are evaluated using the period-mean density relation.